Radio frequency (RF) devices, e.g., RF transistors, and RF power amplifiers (“PAs”) containing RF transistors, are used in a wide variety of communications and other electronic applications, such as cellular handsets and base radio repeaters. RF power amplifiers are typically made up of one or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as the gain stage. Additionally each cascaded amplifier stage can be paralleled with another amplifier to increase the RF power output of the cascaded stage if needed. Ideally, the input to output transfer of each stage is linear, i.e., a perfect replica of the input signal increased in amplitude appears at the amplifier output. In reality, however, all power amplifiers have a degree of non-linearity in their transfer characteristics. This non-linearity is directly related to the level of signal distortion in the amplifier signal output, i.e., the more linear the amplifier, the less distortion is added to the signal.
RF device fault detection is needed so that the end user, e.g., the customer, can be notified that a particular field replaceable unit needs to be replaced. In the event of a paralleled RF device failure in a power amplifier, this notification is needed, for instance, to determine if the product operating with a failed RF device is producing distortion products in excess of requirements imposed by the Federal Communications Commission (“FCC”). If such products exceed FCC limits, the law is being violated and system performance for the user and other system operators will be degraded.
FIG. 1 illustrates a prior art power amplifier 100 configured for enabling the detection of a failed RF device that is included in the amplifier. Power amplifier 100 includes a transistor 110 that in this instance is a metal oxide semiconductor field effect transistor (MOSFET) but may also be some other transistor type such as a Bipolar Junction Transistor (BJT) or a Junction Field Effect Transistor (JFET). Power amplifier 100 also includes: a RF de-coupling device or Radio Frequency Choke (“RFC”) 136 connected to the gate 105 of transistor 110 and a RFC 138 connected to the drain of transistor 110 that are both preferably inductors; a resistor 132 connected between RFC 138 and RFC 136; a resistor 134 connected between a common (ground) at a terminal 130 and the junction of resistor 132 and RFC 136; a resistor 140 connected between a voltage source V+ at a terminal 150 and the junction of resistor 132 and RFC 138; a differential amplifier 142 connected across resistor 140 and having an output swing from substantially close to V+ to substantially close to ground; an RF matching circuit 112 connected to the gate 105 of transistor 110 for receiving an RF input signal (“RFIN”) at a terminal 120; and a RF matching circuit 162 connected to the drain 107 of transistor 110 through which a matched and amplified RF output signal (“RFOUT”) is presented at a terminal 160. In addition, the source of transistor 110 is coupled to an RF common (ground) at terminal 130. Amplifier 100 is illustrated with one transistor for ease of discussion. However, those of ordinary skill in the art will realize that amplifier 100 may, for instance, have cascaded stages and be implemented with additional transistors.
A well-known method for sensing whether transistor 110 has failed is to sense or detect the current being drawn by the drain of transistor 110, for instance, through the generation of an “I-Monitor” signal at a terminal 170. Specifically, a differential voltage is measured across resistor 140 that is proportional to the current being drawn through it and that is a function of the drain current of transistor 110. This differential voltage is then buffered or amplified by the differential amplifier 142, which delivers the single ended (i.e. referenced to ground) “I-Monitor” voltage signal. Accordingly, the “I-Monitor” signal is a single ended voltage representation of the current through resistor 140. The “I-Monitor” signal can then be detected at terminal 170 and compared to a static reference to enable a determination to be made as to whether transistor 110 is operating in a normal state or whether it has failed.
The above method of detecting RF device failure is adequate if the RF device is operating in “class A” because the drain current does not change versus RF drive, thereby enabling the use of a static reference. However, operating in “class A” is usually prohibitive from a utility power and thermal perspective for high power PA's, i.e., amplifiers having an average power out of greater than a few watts. Therefore, most high power PAs operate in “class AB” where the drain or collector currents are proportional to drive. In a class AB operation (as well as with all other classes of operation, e.g., Class B, Class C, etc.) the “I-Monitor” voltage signal will change in approximate proportion to the RF drive since the current through resistor 140 is changing. Thus, with respect to RF devices operating in these latter classes of operations, the current draw in the RF device cannot typically be gauged against a static reference. This means that a static reference cannot typically be used to determine if a failure has occurred.
Therefore for all practical purposes, short of including circuitry in the PA device that detects power out and developing a massive look up table for each RF device (operating in other than Class A) that lists the current drawn as a function of power out, the “health” of the RF device may only be detected using the prior art method during a non-RF drive situation when the RF device is dekeyed so that its quiescent drain current (“Idq”) can be detected and compared to a known value. Ideally, the PA should be frequently dekeyed so that this Idq measurement/comparison can be made. However some PAs operate in a continuously keyed mode, so the Idq measurement/comparison can only occur during maintenance windows, which are typically scheduled a substantial time apart from each other so as not to affect system availability. Thus, if an RF device failed during the time span between maintenance windows, the PA could potentially be operating in an illegal manner for quite some time before an RF device failure is detected.
In addition, this method can be troublesome to implement because resistor 140 may have to be adjusted to a larger value than desired to get a “usable” sensitivity out of differential amplifier 142. This is because operational amplifiers, such as differential amplifier 142, generally have too high of an input offset voltage error to accurately detect the bias level of a transistor. However, raising the value of resistor 140 to increase the sensitivity of differential amplifier 142 would have the adverse effect of causing a lower DC voltage to be coupled to the drain of transistor 110. This would decrease the maximum AC compliance as well as the efficiency of power amplifier 100, which is undesirable. A further shortcoming of detecting the bias level of transistor 110, besides not being very practical, is that it cannot be used with Class C or B designs since these designs do not draw a bias current to begin with.
Thus, there exists a need for a more effective and efficient method for detecting in real-time whether an RF device has failed and that can be used with an amplifier, operating in any class, without having to dekey the amplifier.